Anti-L antibody, raised in sheep with high potassium (HK) red cells by injecting low potassium (LK) red cells is capable of stimulating the Na-K pump rate in LK goat red cells. The stimulation is accomplished by altering the affinity of the LK cell for intracellular potassium; intracellular potassium inhibits the activity of the Na-K pump. The number of antibody molecules bound when the pump is maximally stimulated is about the same as the number of Na-K pumps as determined by tritiated ouabain binding. We hope to determine whether both combining sites of the IgG molecule is necessary to effect pump stimulation. We will prepare monovalent Fab fragments and determine whether or not such fragments are able to stimulate the pump. If they do not stimulate the pump, we will prepare an antibody to the fragments and determine whether reaction of the cells first with the fragments, and then with the antibody, will result in pump stimulation. Further experiments will be undertaken to examine the kinetics of the pump in human red cells. The effect of variations in the concentration of Na and K on one side of the cell on the affinity of the pump for the same ions on the other side of the cell will be examined.